diff --git a/libs/ardour/session.cc b/libs/ardour/session.cc
index 7eb608ff29..f57731b245 100644
--- a/libs/ardour/session.cc
+++ b/libs/ardour/session.cc
@@ -3758,9 +3758,12 @@ Session::io_name_is_legal (const std::string& name)
 
 	for (vector<string>::const_iterator reserved = reserved_io_names.begin(); reserved != reserved_io_names.end(); ++reserved) {
 		if (name == *reserved) {
-			if (route_by_name (*reserved)) {
-				return false;
+			if (!route_by_name (*reserved)) {
+				/* first instance of a reserved name is allowed */
+				return true;
 			}
+			/* all other instances of a reserved name are not allowed */
+			return false;
 		}
 	}